Fluorescent dye-containing beads have been used for many years in diagnostics testing, microscope- and flow-cytometry-based assays, and combinatorial library synthesis. As such they can be routinely manufactured with a variety of surface modifications which enable many classes of molecules to be coupled and subsequently read and manipulated using commercially available instrumentation. The fluorescent organic dye molecules suffer from a number of disadvantages however including photo-bleaching, different excitation irradiation frequencies and broad emissions. Alternatives to conventional fluorescent materials have therefore been investigated. The substitution of the fluorescent organic molecules with luminescent compound semiconductor nanoparticles or “quantum dots” (QDs) is one approach which is intended to circumvent many of these limitations.
The size of a QD dictates the electronic properties of the material; the band-gap energy being inversely proportional to the size of the QDs as a consequence of quantum-confinement effects. Different sized QDs may be excited by irradiation with a single wavelength of light to give a discrete fluorescence emission of narrow band width. Further, the large surface-area-to-volume ratio of the QDs has a profound impact upon the physical and chemical properties of the QD.
Nanoparticles that comprise a single semiconductor material usually have modest physical/chemical stability and consequently relatively low fluorescence quantum efficiencies. These low quantum efficiencies arise from non-radiative electron-hole recombinations that occur at defects and dangling bonds at the surface of the nanoparticle.
Core-shell nanoparticles comprise a semiconductor core with a shell material of typically wider band-gap and similar lattice dimensions grown epitaxially on the surface of the core. The shell eliminates defects and dangling bonds from the surface of the core, which confines charge carriers within the core and away from surface states that may function as centres for non-radiative recombination. More recently, the architecture of semiconductor nanoparticles has been further developed to include core/multishell nanoparticles in which the core semiconductor material is provided with two or more shell layers to further enhance the physical, chemical and/or optical properties of the nanoparticles. To add further stability, a compositionally graded alloy layer can be grown epitaxially on to the nanoparticle core to alleviate lattice strain between adjacent layers that could otherwise lead to defects and reduce the photoluminescence (PL) emission of the QDs. The emission and absorption properties of the QDs can also be manipulated by doping wide band-gap materials with certain metals or luminescence activators to further tune the PL and electroluminescence (EL) at energies even lower than the band gap of the bulk semiconductor material, whereas the quantum size effect can be exploited to tune the excitation energy by varying the size of the QDs without having a significant effect on the energy of the activator-related emission.
The surfaces of core and core/(multi)shell semiconductor nanoparticles often possess highly reactive dangling bonds, which can be passivated by coordination of a suitable ligand, such as an organic ligand compound. The ligand compound is typically either dissolved in an inert solvent or employed as the solvent in the nanoparticle core growth and/or shelling procedures that are used to synthesise the QDs. Either way, the ligand compound chelates the surface of the QD by donating lone pair electrons to the surface metal atoms, which inhibits aggregation of the particles, protects the particle from its surrounding chemical environment, provides electronic stabilisation and can impart solubility in relatively non-polar media.
Various methods have been developed to try to incorporate QDs into beads in the form of resins, polymers, monoliths, glasses, sol gels, silicones, acrylates and other media. The term “beads” is used simply for convenience and is not intended to impose any particular shape or size limitation or composition. It relates to a solid-state medium of any three-dimensional shape of any size and of any composition. For example, beads may be spherical but other configurations are also contemplated.
One of the approaches to incorporating QDs into beads that has been investigated significantly to-date involves pre-coating the QDs with specific types of ligands, which are either polymerisable or compatible with the intended encapsulating polymer. Examples of polymerisation methods that may be used to construct QD-containing beads include suspension, dispersion, emulsion, living, anionic, cationic, RAFT, ATRP, bulk, ring closing metathesis and ring opening metathesis. Initiation of the polymerisation reaction may be caused by any appropriate technique, including free radicals, light, ultrasound, cations, anions, or heat. Examples of polymer-compatible QD surface ligands include ligands that may be hydrophobic, hydrophilic, positively or negatively charged, or functionalised with a reactive group capable of associating with the encapsulating polymer by chemical reaction, covalent linkage, or non-covalent interaction (intercelation). In an alternative approach, QDs have been immobilised in polymer beads through physical entrapment. A solution of QDs in an organic solvent is incubated with a sample of polymer beads. The solvent is then removed, resulting in the QDs becoming immobilised within the matrix of the polymer beads at which point the beads typically require some form of sealing procedure to be carried out to ensure the QDs remain immobilised within the beads. Unfortunately, while progress has been made, most of the approaches currently being investigated to encapsulate nanoparticles require processing steps which can be damaging to the integrity and/or optical performance of the QDs.